The present invention relates generally to the fabrication of photolithography devices such as reticles and semiconductor masks. More particularly, the present invention relates to a method of forming clear fields on a reticle and to reticles formed by electron-beam processing.
In the manufacture of integrated circuits (ICs), microlithography is used to pattern various layers on a wafer. A layer of resist is deposited on the wafer and exposed using an exposure tool and a template, such as a reticle. During the exposure process, radiant energy, such as ultraviolet light, is directed through the reticle to selectively expose the resist in a desired pattern. The resist is then developed to remove either the exposed portions for a positive resist or the unexposed portions for a negative resist, thereby forming a resist mask on the wafer. The resist mask can then be used to protect underlying areas of the wafer during subsequent fabrication processes, such as deposition, etching, or ion implantation processes.
The manufacture of ICs generally requires the use of numerous reticles or masks. Each individual reticle is expensive and time-consuming to manufacture. Mask production likewise involves substantial time and expense. The complete circuit patterning for a typical IC may require 10 to 20 or more reticles. Thus, accurate formation of reticles may yield savings in IC production costs.
Reticles and masks typically include an opaque thin film of a metal, such as chromium or molybdenum silicide, deposited in a pattern on a transparent substrate of quartz or glass. Defects in the pattern of chromium or molybdenum silicide may occur as a result of electrostatic charge added to the reticle preform during manufacture of the reticle. In conventional reticle patterning methodologies, a photoresist material overlays the layer of chromium. An electron beam exposes a portion of the photoresist material based upon a predetermined pattern. The exposed portion of the photoresist material is removed leaving uncovered a portion of the chromium. The unexposed photoresist material is then used to block the etch and leave the desired pattern in the metal to create the reticle.
Referring to FIGS. 1-3, a reticle preform 10 is shown in various stages of manufacture. The reticle preform 10 includes a substrate 12 located on a base 24. The substrate 12 is formed from a transparent material, such as quartz or glass. A layer of metal 13, such as, for example, chromium or molybdenum silicide, overlays the substrate 12 and is located beneath a layer of a photoresist material 14. The photoresist material 14 is formed of a material which is suitable for exposure by electrons.
An electron beam apparatus 16 is schematically shown (FIGS. 1, 2) in a position to direct electrons toward the photoresist material 14. The apparatus 16 includes an electron beam device 18, such as an electron beam gun, in mechanical and electrical connection with a controller 22. An actual electron beam gun, such as one manufactured by ETEC systems, is illustrated in FIG. 10. The electron beam device 18 directs a stream of electrons 26 toward the photoresist material 14 in a predetermined writing pattern 28, shown by the dashed lines on the photoresist material 14. The stream of electrons 26 preferably is controlled electrostatically.
Conventionally, a single predetermined writing pattern 28 is programmed into the controller 22, which controls the actions of the electron beam device 18 through the appendage 20. The writing pattern 28 is followed such that predetermined portions of the photoresist material 14 are exposed by the stream of electrons 26. The exposed portions of the photoresist material 14 are then removed. The remaining unexposed portions of the photoresist material 14 are used as a mask for etching the now exposed portions of the metal 13 to create a reticle 100 (FIG. 3) having the desired pattern of metal 13.
Specifically, and with reference to FIGS. 1 and 2, the writing pattern 28 separates the photoresist material 14 into a first strip 30, a second strip 32, a third strip 34, a first portion 36, a second portion 40, a third portion 70, and an interlayer portion 68. In FIG. 1, the writing pattern 28 is shown in dashed lines to indicate that the exposure process has only just started. In FIG. 2, the writing pattern 28 is shown in solid lines to indicate that the exposure process has been completed.
In the known process, the stream of electrons 26 exposes the portions 36, 40, 70, and 68 allowing for the subsequent removal of the photoresist material 14 resident in the exposed areas. One problem encountered through the conventional methodology is that using an electron beam to expose large photoresist areas, such as the second and third portions 40, 70, sometimes causes a localized build up of electrostatic energy in the reticle preform 10. The presence of electrostatic energy is detrimental to the accuracy of the stream of electrons 26, causing the stream 26 to be displaced, or to skew away, from the path intended by the writing pattern 28 (FIG. 2), thus altering the pattern of exposed photoresist material 14 from the desired writing pattern 28.
Applicant has determined that where electrostatic energy has caused a displacement of the electron stream 26 the photoresist material 14 exposed may not be consistent with the amount intended to be exposed according to the writing pattern 28. Instead, the exposed photoresist material 14 which is subsequently removed will leave first, second, third, and fourth uncovered areas of metal 56, 42, 71, and 69 which respectively were beneath the portions 36, 40, 70, and 68. Since the exposed portions 36, 40, 70, and 68 did not exactly correspond with the writing pattern 28, the underlying metal areas 56, 42, 71, and 69 also will not match the desired metal areas according to the writing pattern 28. In addition, the remaining unexposed portions of photoresist material 14, namely a first strip 50, a second strip 52 and a third strip 72 do not match with the unexposed strips that were to be formed according to the intended writing pattern 28.
After removing the exposed photoresist material 14 (as described above), the exposed areas of metal, namely the first, second, third and fourth uncovered areas of metal 56, 42, 71, 69 are etched. The remaining unexposed portions of photoresist, namely the first, second, and third strips 50, 52, 72 are washed away by a known method to form the reticle 100 (FIG. 3), including metal strips 62, 64, 66 positioned on the substrate 12.
Since the exposed and unexposed portions of the photoresist material 14 did not match the writing pattern 28, the metal strips 62, 64, 66 will likewise differ from the desired strips. The discrepancy between the actual metal strips 62, 64, 66 and the desired strips may be substantial enough to cause the reticlc 100 to form defective semiconductor devices. Alternatively, additional measures may be required to compensate for the discrepancy.
The present invention provides a method of forming a reticle including exposing a first portion of the photoresist layer in accordance with a first writing pattern and exposing a second portion of the photoresist layer in accordance with a second writing pattern.
The present invention also provides a photolithography device for forming a semiconductor device that has a transparent substrate and a pattern of conductive material overlaying the substrate. The conductive material pattern is formed utilizing multiple write passes of electron beam energy.
The present invention also provides an apparatus for forming a photolithography device. The apparatus includes a device for projecting electrons at a layer of photoresist material and a controller for controlling the device such that a multiple of write passes based upon corresponding patterns sequentially expose portions of the photoresist material.
These and other advantages and features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings.